1. Field of the Invention
This invention relates generally to a toroidal ion trap mass analyzer that may be used in the creation, storage, separation and analysis of ions according to mass-to-charge ratios of changed particles and charged particles derived from atoms, molecules, particles, sub-atomic particles and ions. More specifically, the present invention is a novel design of the toroidal ion trap mass analyzer that may be easier and less expensive to manufacture, lends itself to miniaturization, and performs as well or better than existing toroidal ion trap designs.
2. Description of Related Art
Mass spectrometry continues to be an important method for identifying and quantifying chemical elements and compounds in a wide variety of samples. High sensitivity and selectivity of mass spectrometry are especially useful in threat detection systems (e.g. chemical and biological agents, explosives) forensic investigations, environmental on-site monitoring, and illicit drug detection/identification applications, among many others. Thus, the need for a reliable mass analyzer that can perform in-situ makes a portable device even more relevant. Some key elements in developing portable mass spectrometers are reduction in size, weight and power consumption, along with reduced support requirements and cost.
Ion trap analyzers are inherently small, even as implemented commercially. Ion trap analyzers also have only a few ion optic elements, which do not require highly precise alignment relative to other types of mass analyzers. In addition, because they are trapping devices, multiple stages of mass spectrometry (MS) can be performed in a single mass analyzer. The operating pressure for ion traps is higher than other forms of mass spectrometry allowing for less stringent pumping requirements. Furthermore, because the radio frequency (RF) trapping voltage is inversely proportional to the square of the analyzer radial dimension, a modest decrease in analyzer size results in a large reduction in operating voltage. This in turn results in lower power requirements. An added potential benefit of the reduced analyzer size is the shorter ion path length which may ease the vacuum requirements even further. As a practical matter, the shorter ion path length is especially important as some of the most limiting aspects of MS miniaturization are not in the ion optic components, but rather in the vacuum and other support assemblies.
FIG. 1 is a perspective view of a toroidal ion trap 10 as found in the prior art. FIG. 2 is a cross-sectional view of the toroidal ion trap 10 of FIG. 1. Notice that the outer ring assembly 12 and the inner ring assembly 14 are hyperbolic surfaces, as well as the end-caps. These hyperbolic surfaces are difficult to machine in small dimensions. The trapping volume is shown as the dotted circle 16. The trapping volume 16 becomes a circular ring when seen from above as the toroidal ion trap 10 is rotated around a central axis 18.
The toroidal ion trap suffers from several inherent field defects. Field defects are irregularities or non-uniformities in the electric fields generated in toroidal ion traps that make it difficult or impossible to make electrical fields that are perpendicular to ion motion at all values of z (the axial variable) during axial ion ejection. The result is that as ions are ejected they are also pushed in radial directions, distorting the ion packet and compromising either sensitivity, resolution, or both.
The problem above is illustrated in FIGS. 3A and 3B. In FIG. 3A, the electrodes 20 and 22 are shown as being symmetric. The problem is that the ejection path 24 for ions is not along a path 26 that is perpendicular to the electric field shown as field lines 28. The ions are being directed into the walls of the toroidal ion trap 10 instead of along the desired ejection path 24 that leads to a detector (not shown).
The hyperbolic electrode surfaces in FIG. 3B are required to correct the shape of the electric fields in order to have ejection of ions in a desired direction. Furthermore, the electrodes do not even have the same hyperbolic shape. It is also noted that the RF electrode end-caps also have hyperbolic surfaces.
Another problem with conventional toroidal ion traps is that the hyperbolae of revolution
are quite complex. This complexity makes the toroidal ion trap difficult to machine. This difficulty gets even harder when done on a small scale, making them hard to miniaturize.
There are several advantages over existing ion traps when using a miniaturized design. First, there is a relaxed vacuum requirement. The result is reduced pump power needed to create the vacuum inside the toroidal ion trap. Another advantage is the reduced instrument power required to operate the toroidal ion trap. Another advantage is the reduced weight which is especially important in mobile or field applications. Thus, the desire to miniaturize a toroidal ion trap raises the issue of how to simplify the design so that a miniature toroidal ion trap can be more easily manufactured than current hyperbolic electrode designs.
It is also important to understand that as these devices become smaller, the machining tolerances play an increasingly significant role in trapping field defects. Thus, it would be an advantage over the prior art to simplify the geometry of the walls that function as electrodes to have a design that is more easily machined.
Cylindrical ion trap mass analyzers 40 as shown in FIG. 4 have been miniaturized because the simplified, straight lines or a cylinder are considerably easier to machine than hyperbolic surfaces, especially in small dimensions. When the geometry of the ion trap electrodes deviates significantly from the theoretical geometry, as is the case for cylindrical ion traps 40, corrections are needed to restore the trapping field potentials to their theoretical values. Modeling and simulation programs have been used extensively in this undertaking.
Disadvantageously, the gains from reducing analyzer size (e.g. increased portability due to lower weight and smaller size, lower RF generator power, and relaxed vacuum requirements) are understandably offset by a reduction in ion storage capacity in state of the art mass analyzers.
Attempts have been made to recover the lost ion storage capacity in miniaturized ion trap designs. For example, arraying several reduced volume cylindrical ion traps is one approach. More recently, linear ion traps with either radial or axial ejection have also been developed. The increased ion storage capacity is due to the volume available throughout the length of the two-dimensional quadrupole rod array. These devices are now readily available in commercial versions.
Toroidal ion traps cannot easily be reduced further in size without deterioration in electric field shape and its corresponding performance. Using cylindrical-shaped electrodes to approximate a toroidal ion trap would seem to be an obvious approach to miniaturization, but it cannot be done in an obvious manner due to fundamental hyperbolic electrode shapes that exists in conventional toroidal ion trap designs.
The toroidal ion trap geometry offers some unique advantages as a miniature mass analyzer if it can be designed. All ions are contained within a single trapping field so, unlike arrays, there is no concern in matching the individual arrays or in interfacing ion sources or detectors to ensure equal illumination or sampling from each cell of the array. In fact, the circular form offers a compact geometry which can be easily interfaced to ionizers and electron multiplier detectors.
Finally, in contrast to conventional linear quadrupole ion traps, the trapping field is homogeneous throughout the entire trapping volume (i.e. there are no end effects because the trapping volume is annular) and all ions of a given mass-to-charge ratio (m/z) are simultaneously ejected.